1. Technical Field
This invention relates to selectively permeable membranes and, more particularly, to polymerized silicon-containing compounds and oxygen sensing electrodes coated with such membranes.
2. Background
The use of permeable membranes for separating gases or other substances is based upon the selective permeability of certain organic materials. The terms "selective permeability" or "permselective" mean that one component will permeate through a membrane faster than other components of a mixture. The terms do not imply that the passage of one component occurs with the complete exclusion of others. Instead, they indicate that the difference in the flow rate (i.e., permeability) of two different molecular components through a membrane is based upon each component's solubility in and diffusion rate through that membrane. The mechanism results in large differences in permeation rates for the same component through different membranes as well as for different components in a given membrane.
Permselective membranes have been developed to separate and purify both liquid and gas mixtures. Permselective membranes can be used on a commercial scale for liquid-liquid or liquid-solid separation such as the conversion of sea water to fresh water. And, certain membranes can be used for gas-gas separation to obtain high purity gas. Gas permselective membranes also can be used in making oxygen sensing devices, such as oxygen electrodes.
An oxygen electrode can be used as a cathode in an oxygen sensing device for the reduction of oxygen in a sample environment. Oxygen electrodes can be used to measure the oxygen concentration of a gas or of a biological environment, such as the in vivo measurement of oxygen in blood. The electrode provides a current or voltage output which is a function of the oxygen concentration in that environment.
Conventional electrodes are constructed of conductive metals such as platinum or gold which catalyze the following cathodic reduction reaction. ##STR1##
In certain environments, the electrode can rapidly lose its ability to catalyze the second step of the four electron reduction reaction. In addition, certain ions or proteins are adsorbed onto the electrode, and this fouling or contamination reduces the predictability and reproducibility of the electrode's operation. To avoid these problems, the electrodes can be protected by a permselective membrane coating.
The membrane separates the conductive metal from the environment in which it is used. The membrane's selective permeability permits oxygen to reach the electrode, so that the environmental oxygen can be measured, but prevents unwanted components from fouling the electrode.
Permeable membranes are conventionally made by dip coating and casting methods. In the casting method, an appropriate solvent is used to dissolve the membrane precursor material, and the mixture is cast onto a solid surface for drying. The solvent is removed, by heat or vacuum evaporation, leaving a membrane on the surface. The membrane then can be removed from the substrate and attached to the surface of the desired support such as an electrode. The dip method is similar, except that the substrate material is immersed in the membrane precursor material.
Problems with the dip and casting techniques occur because the permeable membrane generally is not formed on the material that will serve as the final support. Thus, the shape of the membrane may not conform identically to the support material, and typically the adhesion between the membrane and the surface of the final support material is poor. Furthermore, only materials with large surfaces are readily covered with a membrane by the casting method because the fabricated membrane must be removed from the casting surface and reapplied to the final support material. Another disadvantage is that the thickness of the membrane is difficult to control because the solvent must be evaporated from the mixture to form the membrane. Evaporation introduces variations in membrane thickness which can alter the membrane's permselective properties.
In an effort to avoid problems with adhesion, conformity, uniformity and substrate size, plasma polymerized membranes have been developed, mainly for use as reverse osmosis membranes or gas permeable membranes. Semipermeable membranes have been made using glow-discharge polymerization techniques, and such membranes have been adapted for use as electrode coatings. For example, aliphatic hydrocarbons have been plasma polymerized to form semipermeable membranes. The resultant membranes, however, do not have the highest selectivity for permeation by oxygen. Thus, such membranes are not the most satisfactory coatings for oxygen sensing electrodes. Tertiary organic silicon-containing compounds have been used together with both silicon rubber and a membranous substrate to form a semipermeable laminated membrane for use in gas-gas separations. For example, a silicon-containing compound such as silicon rubber is used to fill the pores of a porous polymeric membrane, one side of the membrane is exposed to an unpolymerizable gas plasma to cross-link the siloxane within the pores, and then a second membrane is laminated to that surface by plasma polymerization of a tertiary silicon compound. While the silicon-containing laminated membrane has a higher selectivity for oxygen than does the aliphatic hydrocarbon membrane, the process required starting with a porous membranous substrate and laminating or depositing a second membrane onto it. Because the porous membranous substrate is not formed on the material that will serve as the final support, such as an electrode, the advantages of plasma polymerization are lost, and the thickness of the final laminated membrane is controlled by the thickness of the initial porous substrate membrane.